A perchlorate ion (ClO4−), a tetrafluoroborate ion (BF4−), a nitrate ion (NO3−), and a bromide ion (Br−) have high solubility in water, although being harmful ions, and are therefore anions that are difficult to remove from aqueous solutions. In addition to the anions, an iodide ion (I−) also has high solubility in water and is an anion that is difficult to remove from aqueous solutions.
For example, it has been feared that perchlorate ions cause infants and children to develop growth impairment or intellectual impairment by being ingested in excess.
As a method for removing an anion such as a perchlorate ion from an aqueous solution, for example, a method using an ion exchange resin has been known (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2004-346299 and “NEDO Foreign Report, No. 946, 2004.12.15”).
Further, there has been known a method of producing a trapping capsule type molecule with a structure in which a perchlorate ion is encapsulated in a capsule framework consisting of four molecules of bitb and two Cu2+ ions by adding 1,4-bis(imidazol-1-yl-methyl)2,3,5,6-tetramethylbenzene (hereinafter also referred to as “bitb”) to an aqueous solution containing the perchlorate ion and precipitating the produced trapping capsule type molecule (for example, see International Publication No. WO 2008/029804 pamphlet). It has been known that bitb exhibits the effect of removing other anions such as a tetrafluoroborate ion (BF4−) as well as the perchlorate ion (ClO4−) (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2010-022886 and Japanese Patent Application Laid-Open (JP-A) No. 2010-042403).